1. Field of Invention
This invention relates generally to semiconductor memories and specifically to content addressable memories.
2. Description of Related Art
Content addressable memories (CAMs) are frequently used for address look-up functions in Internet data routing. For example, routers used by local Internet Service Providers (ISPs) typically include one or more CAMs for storing a plurality of Internet addresses and associated data such as, for instance, corresponding address routing information. When data is routed to a destination address, the destination address is compared with all CAM words, e.g., Internet addresses, stored in the CAM array. If there is a match, routing information corresponding to the matching CAM word is output and thereafter used to route the data.
A CAM device includes a CAM array having a plurality of memory cells arranged in an array of rows and columns. Each memory cell stores a single bit of digital information, i.e., either logic zero or logic one. The bits stored within a row of memory cells constitute a CAM word. During compare operations, a comparand word is received at appropriate input terminals of a CAM device and driven into the CAM array using comparand lines to be compared with all the CAM words in the device. For each CAM word that matches the comparand word, a corresponding match line signal is asserted to indicate a match condition. If the comparand word matches more than one of the CAM words, the match line corresponding to each of the matching CAM words is asserted, and a "multiple match" flag is also asserted to indicate the multiple match condition. The match line signals from each CAM block are combined in a priority encoder to determine the index or address of the highest-priority matching CAM word. Associative information corresponding to the highest-priority matching CAM word stored in, for instance, an associated RAM, may also be provided.
A single CAM device can be used to store multiple tables each storing and maintaining different classes of data. All entries, however, typically participate in a compare operation. This can cause an undesirable amount of power to be drawn during the compare operation. It would be desirable to limit a search to only those entries associated with a particular class of data to reduce power consumption during the operation.